Non-contact electromagnetic tracking systems are well known in the art, with a wide range of applications.
U.S. Pat. No. 5,391,199, to Ben-Haim, whose disclosure is incorporated herein by reference, describes a system for generating three-dimensional location information regarding a medical probe or catheter. A sensor coil is placed in the catheter and generates signals in response to externally-applied magnetic fields. The magnetic fields are generated by three radiator coils, fixed to an external reference frame in known, mutually-spaced locations. The amplitudes of the signals generated in response to each of the radiator coil fields are detected and used to compute the location of the sensor coil. Each radiator coil is typically driven by driver circuitry to generate a field at a known frequency, distinct from that of the other radiator coils, so that the signals generated by the sensor coil may be separated by frequency into components corresponding to the different radiator coils.
U.S. Patent Application Publication US 2002/0065455 A1, to Ben-Haim et al., whose disclosure is incorporated herein by reference, describes a system that generates six-dimensional position and orientation information regarding the tip of a catheter. This system uses a plurality of sensor coils adjacent to a locatable site in the catheter, for example near its distal end, and a plurality of radiator coils fixed in an external reference frame. These coils generate signals in response to magnetic fields generated by the radiator coils. The strengths of the signals generated in the sensor coils due to each of the different radiator coils are input to a system of non-linear algebraic equations, which are solved by numerical approximation to compute six location and orientation coordinates of the catheter.
Other locating devices using a position sensor attached to a catheter are described, for example, in U.S. Pat. No. 6,239,724 to Doron et al., U.S. Pat. No. 5,425,382 to Golden et al., U.S. Pat. No. 5,558,091 to Acker et al., U.S. Pat. No. 4,173,228 to Van Steenwyk et al., U.S. Pat. No. 5,099,845 to Besz et al., U.S. Pat. No. 5,325,873 to Hirschi et al., U.S. Pat. No. 5,913,820 to Bladen et al., U.S. Pat. No. 4,905,698 to Strohl, Jr. et al., and U.S. Pat. No. 5,425,367 to Shapiro et al. The disclosures of these patents are incorporated herein by reference. Commercial electrophysiological and physical mapping systems based on detecting the position of a probe inside the body are presently available. Among them, CARTO™, developed and marketed by Biosense Webster, Inc. (Diamond Bar, Calif.), is a system for automatic association and mapping of local electrical activity with catheter location.
The above-described tracking systems generally rely on separation of position-responsive signals into components, most typically frequency components. Each such component is assumed to correspond uniquely to a single radiator coil, in a known position, radiating a magnetic field having a regular, well-defined spatial distribution. In practice, however, when a metal or other magnetically-responsive article is brought into the vicinity of the catheter or other object being tracked, the magnetic fields in this vicinity are distorted. In a surgical environment, for example, there can be a substantial amount of conductive and permeable material, including basic and ancillary equipment (operating tables, carts, movable lamps, etc.), as well as invasive surgery apparatus (scalpels, catheters, scissors, etc.) The magnetic fields of the radiator coils may generate eddy currents in such articles, and the eddy currents then cause a parasitic magnetic field to be radiated. Such parasitic fields and other types of distortion can lead to errors in determining the position of the object being tracked.
Various methods are known in the art for detecting and compensating for the presence of magnetically-responsive articles in the field of a magnetic tracking system. For example, U.S. Pat. No. 6,147,480, to Osadchy et al., whose disclosure is incorporated herein by reference, describes a method for tracking an object using energy fields, in the presence of interference due to introduction of an article that is responsive to the fields. Energy fields are produced in the vicinity of the object, and a characteristic, such as a phase shift, of the parasitic energy fields induced due to introduction of the article is determined. This characteristic is then used in processing signals generated in response to the energy field at different locations of the object, in order to determine spatial coordinates of the object.
U.S. Pat. No. 6,373,240, to Govari, whose disclosure is incorporated herein by reference, describes an object tracking system comprising one or more sensor coils adjacent to a locatable point on an object being tracked, and one or more radiator coils, which generate alternating magnetic fields in a vicinity of the object when driven by respective alternating electrical currents. The frequencies are scanned through a plurality of values such that at any given time, each of the radiator coils radiates at a frequency which is different from the frequencies at which the other radiator coils are radiating. The sensor coils generate electrical signals responsive to the magnetic fields, which are perturbed by parasitic field components due to field-responsive articles in the vicinity of the object. The signals are analyzed to find an optimal frequency, at which the perturbing effect of the parasitic components is minimized. The optimal frequency is used in detecting spatial coordinates of the object.
U.S. Pat. No. 6,172,499, to Ashe, whose disclosure is incorporated herein by reference, describes a device for measuring the location and orientation of a receiving antenna with respect to transmitting antennas using multiple-frequency AC magnetic signals. The transmitting component consists of two or more transmitting antennas of known location and orientation relative to one another. The transmitting antennas are driven simultaneously by AC excitation, with each antenna occupying one or more unique positions in the frequency spectrum. The receiving antennas measure the transmitted AC magnetic field plus distortions caused by conductive metals. A computer then extracts the distortion component and removes it from the received signals, providing the correct position and orientation output.
U.S. Pat. No. 5,767,669, to Hansen et al., whose disclosure is incorporated herein by reference, describes a method for subtracting eddy current distortions produced in a magnetic tracking system. The system uses pulsed magnetic fields from a plurality of generators. The presence of eddy currents is detected by measuring rates of change of currents generated in sensor coils used for tracking. The eddy currents are compensated for by adjusting the duration of the magnetic pulses.
European Patent Application EP 0 964261 A2, to Dumoulin, whose disclosure is incorporated herein by reference, describes systems for compensating for eddy currents in a tracking system using alternating magnetic field generators. In a first system the eddy currents are compensated for by first calibrating the system when it is free from eddy currents, and then modifying the fields generated when the eddy currents are detected. In a second system the eddy currents are nullified by using one or more shielding coils placed near the generators.
U.S. Pat. No. 6,369,564, to Khalfin et al., whose disclosure is incorporated herein by reference, describes an electromagnetic tracking system that includes at least one source of an AC electromagnetic field, at least one witness sensor measuring components of the electromagnetic induction vector at known spatial points close to or within the volume of interest, and at least one wireless probe sensor placed on the object being tracked. The signal generated by the witness sensors is used in separating environmental distortion signals from the probe sensor signal, by distinguishing the phase of the signal from the probe sensor.